Python

Index

1. Scripts revisited

• the #!/bin/bash found at the beginning of a script, for example

• #!/bin/bash
  
  echo "hello world"

• indicate to GNU/Linux & UNIX that this is a bash script and that the script must be run using this interpreter
  

2. Effect of #! on GNU/Linux and Cygwin

• under GNU/Linux and Windows (Cygwin) it is treated as a comment by the bash interpreter but, critically,
• the GNU/Linux operating system exec routine sees #! as a 16 bit magic number indicating that this file must be run with the program found in the string up to the next newline
  • thus should a script be given executable permission it will behave to users like an executable binary
• #! means nothing to Windows and so the bash interpreter on Windows or (cygwin) simply treats this line as a comment.

3. Autoftp in python

• consider an autoftprc file which consists of a number of ftp url’s

• # GNU Troff
  floppsie.comp.glam.ac.uk/pub/groff/groff.tar.gz

• a # indicates a comment
• step one in creating the autoftp script in python is to read in the autoftprc file a line at a time
• step two, remove the comments (hint examine split)
  

4. Autoftp in python

• step three, split the ftp url into 3 components
  • site
  • directory
  • filename
• step four, retrieve the file (hint examine the ftp module)

5. Reading a file, a line at a time

• #!/usr/bin/python
  
  
  for line in open(’autoftprc’).readlines():
      print line

• removing a comment can be achieved by:

• uncomment = string.split(line, ’#’)
  print uncomment[0]

6. Extracting the site, directory and filename

• think split again

• words = string.split(uncomment[0], ’/’)

  • words[0] is the site
  • words[1..n-2] is the directory
  • words[-1] is the filename
  • n is len(words)
    

7. Putting the components together

• finally here is the autoftp program in python so far:

• #!/usr/bin/python
  
  import sys, string
  from ftplib import FTP
  
  for line in open(’autoftprc’).readlines():
      uncomment = string.split(line, ’#’)
      words = string.split(uncomment[0], ’/’)
      i = len(words)
      if i>1:
           print  ’ is the site’, words[0]
           print  ’ is the file’, words[i-1]
           directory=""
           for j in range(1,i-1):
              directory = directory + ’/’ + words[j]
           site = words[0]
           file = words[-1]
  
           print ’directory is’, directory

8. Turorial work

• copy the autoftprc file as given in these slides (make sure you call your file autoftprc )
• now create a Python script called autoftp.py and cut and paste the code from stage 1 into this file.
• run this code and satisfy yourself that you understand what is going on
• now move to stage 2 and print all the uncommented lines in autoftprc
• now split the url into the three components: site, directory and filename
  • hint solve the site and filename first and then solve the directory.
  • you should just print these items to the screen
• now read the ftp module in the global module index (found in the Python documentation)
• import the ftp module and call the appropriate methods necessary to download the file from the remote site

Index

This document was produced using groff-1.19.